Field of the Invention
The invention relates to a measuring device for testing unpacked chips, in particular IC circuits. The measuring device includes a needle-board circuit board and a large number of contact-making needles, which are carried by the needle-board circuit board, for making contact with connecting areas (pads) belonging to an IC circuit. The measuring device also includes lines for feeding test signals, at a clock frequency in the MHz range or a higher frequency, to the contact-making needles and for carrying measured signals away from the contact-making needles. The contact-making needles have a shank part with a length l2 and a tip part, adjacent thereto, with a length l1. Each of the shank parts of the contact-making needles contacts a section of the conductor tracks. Such a measuring device is disclosed in U.S. Pat. No. 5,594,358.
In the development of chips, for example in the form of IC circuits, a testing process is in widespread use in which, signals are applied simultaneously to a large number of connecting areas, so-called pads, and an equally large number of measured signals are tapped off. These signals are applied at a clock frequency that is normally several hundred megahertz by means of high-speed electronic interfaces. In order to transmit the signals to the pads and to tap off measured signals from the pads, a needle-board circuit board is used which has a large number of contact-making needles to make contact with the pads. Hitherto, the contact-making needles have typically had a length of about 15 to 40 mm. For transmitting signals, wire lines are used which are connected to the high-speed interfaces and are connected, via wiring tracks on the needle board, to the shank parts of the contact-making needles.
The problem with these known measuring devices is that the wire lines do not ensure optimal impedance matching when high-frequency technology is involved. This means that the contact-making needles are not matched in terms of impedance to the 50-xcexa9 characteristic impedance of the other feed network. Because of this mismatching, there is a discontinuity in the transmission of the high-frequency waves, which generally leads to reflections and therefore to attenuation of the transmitted waves. A further source relating to signal impairment, specifically to signal attenuation, lies in the configuration of solder points, which have hitherto been used to connect the contact-making needles to the needle-board circuit board. This source of interference constitutes a geometric discontinuity in the waveguide arrangement of the signal transmission and leads to attenuation of the signals. In addition, the measuring devices used hitherto do not permit any mass shielding of the signal lines, and therefore, the previous measuring devices are sensitive to coupled in interference signals, in particular to signal crosstalk. In addition, when the lengths of the needles are unfavorable in relation to the wavelengths of the transmitted signals, the contact-making needles radiate signal power and likewise contribute to attenuation of the transmitted signals. A further source of signal interference is the parasitic tuned circuit which necessarily results from the effective needle inductance and the input capacitance of a chip pad, plus the connected circuits.
It is accordingly an object of the invention to provide a measuring device for testing unpacked chips which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type. In particular, it is an object of the invention to provide a measuring device that permits more accurate signal measurement and optimal interference signal suppression.
With the foregoing and other objects in view there is provided, in accordance with the invention, a measuring device for testing unpacked chips that includes a needle-board circuit board carrying a plurality of contact-making needles for making contact with connecting areas on an IC circuit. The needle-board circuit board has lines for feeding test signals with a clock frequency that is at least in a megahertz range to the plurality of the contact-making needles. These lines are also for carrying measured signals away from the plurality of the contact-making needles. Each one of the plurality of the contact-making needles includes a shank part having a length l2 and a tip part having a length l1. The tip part is adjacent the shank part. Each one of the lines has a section making contact with the shank part of a respective one of the plurality of the contact-making needles. The lines are formed as dual-transmission conductor tracks on the needle-board circuit board. Each one of the plurality of the contact-making needles has an overall length such that l1+l2 less than xcex/2, where xcex is a maximum wavelength of the signal that is transmitted via a respective one of the lines.
Accordingly, the invention provides for transmission-line wiring in combination with a low-impedance contact-making needle in which these needles directly contact the transmission-line lines. The transmission-line wiring ensures extremely short feed lines from the needle board to the contact pads of a chip or IC, so that interference, which is caused by excessively long feed lines, is largely suppressed.
The xe2x80x9cshortxe2x80x9d needle in the sense of the invention, in which the overall needle length of the contact-making needles is less than the wavelength of the transmitted signals, provides the advantage that a 50-xcexa9 characteristic impedance is ensured in the area of the contact-making needles corresponding to that of the feed-line network. The combination of a short needle and a transmission-line signal supply line to the needle permits signal transmission to an IC contact pad and away from the latter with an optimum impedance and therefore with the lowest possible attenuation.
In order to optimize the impedance matching, the overall length of the contact-making needles is selected to be considerably smaller than a quarter wavelength of the transmitted signals. In view of the frequency that is typically transmitted, including harmonics, up to the region of 1 to 10 GHz, the overall length of the contact-making needles is preferably about 5 mm.
The invention envisages a plurality of possible ways for connecting the contact-making needles to the conductor tracks of the needle-board circuit board, as indicated below.
According to one connection variant, each line, which is provided in the form of a conductor track on the needle-board circuit board, has a signal feed part on one side of the needle-board circuit board and a measured signal discharge part on the other side of the needle-board circuit board. The signal feed part is connected to the signal discharge part of the line via a contact-making needle connecting part. This contact-making needle connecting part runs over the narrow side of the needle-board circuit board and is in the form of a continuation of the conductor tracks on the two needle-board circuit board surfaces.
Alternatively, each line has a signal feed part on one side of the needle-board circuit board and a measured signal discharge part on the other side of the needle-board circuit board in which the signal feed part is connected to the signal discharge part of the line via a contact-making needle connecting part, however, this contact-making needle connecting part runs through a contact via in the needle-board circuit board. The shank part of a contact-making needle is inserted into a corresponding one of the vias.
In order to optimize the low level of interference in the signal transmission by means of the dual transmission conductor tracks, the invention provides for a ground line to be arranged between the signal feed part and the measured signal discharge part of the line. The ground line ends with its free end at a distance C from the shank part of the contact-making needle. This distance C is related to the characteristic impedance and the length l2 as follows:
w=L2/C,
L2 being the inductance of the contact-making needle, C being the capacitance to ground of the conductor track to which the contact-making needle is connected;
L2=xcexcxc3x97xcexc0xc3x97l2
where xcexcis the magnetic permeability of the needle material, and xcexc0 is the magnetic field constant; and
C=xcex5xc3x97xcex50xc3x97bxc3x97l2/c
where xcex5 is the dielectric constant of the needle circuit board material, xcex50 being the permitivity and b being the width of that conductor track to which the contact-making needle is connected.
Here, the distance c should be selected such that the characteristic impedance w agrees exactly as possible with the characteristic impedance in the dual-transmission conductor area. The ground line can be provided in sandwich fashion between the signal feed part and the signal discharge part, and can be in the shape of an at least approximately equally wide conductor track. As an alternative to this, the ground line can be arranged as a continuous ground plane between the signal feed parts and the measured signal discharge lines of a large number of dual-transmission conductor tracks located beside one another.
In addition to the ground line indicated above, ground lines can be arranged between the dual-transmission conductor tracks running beside one another that make contact with a large number of contact-making needles. This, too, contributes to interference-free signal transmission.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a measuring device for testing unpacked chips, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.